/*
 * $Header$
 * $Revision$
 * $Date$
 *
 * ====================================================================
 *
 * Copyright 2000-2002 bob mcwhirter & James Strachan.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 * 
 *   * Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 * 
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 * 
 *   * Neither the name of the Jaxen Project nor the names of its
 *     contributors may be used to endorse or promote products derived 
 *     from this software without specific prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
 * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER
 * OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * ====================================================================
 * This software consists of voluntary contributions made by many 
 * individuals on behalf of the Jaxen Project and was originally 
 * created by bob mcwhirter <bob@werken.com> and 
 * James Strachan <jstrachan@apache.org>.  For more information on the 
 * Jaxen Project, please see <http://www.jaxen.org/>.
 * 
 * $Id$
 */


package org.jaxen;

/**
 * Resolves variable bindings within an XPath expression.
 *
 * <p>
 * Variables within an XPath expression are denoted using
 * notation such as <code>$varName</code> or
 * <code>$nsPrefix:varName</code>, and may
 * refer to a <code>Boolean</code>, <code>Double</code>, <code>String</code>,
 * node-set (<code>List</code>) or individual XML node.
 * </p>
 *
 * <p>
 * When a variable is bound to a node-set, the
 * actual Java object returned should be a <code>java.util.List</code>
 * containing XML nodes from the object-model (e.g. dom4j, JDOM, DOM, etc.)
 * being used with the XPath.
 * </p>
 *
 * <p>
 * A variable may validly be assigned the <code>null</code> value,
 * but an unbound variable (one that this context does not know about)
 * should cause an {@link UnresolvableException} to be thrown.
 * </p>
 *
 * <p>
 * Implementations of this interface should implement <code>Serializable</code>.
 * </p>
 *
 * @author <a href="mailto:bob@werken.com">bob mcwhirter</a>
 * @author <a href="mailto:jstrachan@apache.org">James Strachan</a>
 * @see SimpleVariableContext
 * @see NamespaceContext
 */
public interface VariableContext
{
    /**
     * An implementation should return the value of an XPath variable
     * based on the namespace URI and local name of the variable-reference
     * expression.
     *
     * <p>
     * It must not use the prefix parameter to select a variable,
     * because a prefix could be bound to any namespace; the prefix parameter
     * could be used in debugging output or other generated information.
     * The prefix may otherwise be ignored.
     * </p>
     *
     * @param namespaceURI  the namespace URI to which the prefix parameter
     *                       is bound in the XPath expression. If the variable
     *                       reference expression had no prefix, the namespace
     *                       URI is <code>null</code>.
     * @param prefix        the prefix that was used in the variable reference
     *                       expression; this value is ignored and has no effect
     * @param localName     the local name of the variable-reference
     *                       expression. If there is no prefix, then this is
     *                       the whole name of the variable.
     * @return  the variable's value (which can be <code>null</code>)
     * @throws UnresolvableException  when the variable cannot be resolved
     */
    public Object getVariableValue( String namespaceURI,
                                    String prefix,
                                    String localName )
        throws UnresolvableException;
}
